SOLVED:
I tried to create a new user and see if it works from there- it did.
After some trial and error with copying and overwriting the contents of the .config folder I’ve managed to find the problem- plasma-localerc.
My plasma-localerc detailed settings (time, currency etc.), the default didn’t. I removed everything except:
[Formats]
LANG=en_US.UTF-8
And it worked.
TL;DR: Deleting the locales but LANG in the ~/.config/plasma-localerc file solved the problem.